This invention relates to a method and apparatus for microcomputer control of an engine, and, more particularly, to a method and apparatus for controlling an air-fuel ratio to a motor vehicle engine wherein an amount of fuel supplied to the engine is controlled relative to an amount of suction air in the engine.
In a conventional engine controlling method using a microcomputer, various different sensors supply data of operating conditions of engine, on which the basic amount of fuel supplied is determined and controls the carburetor or fuel injector through the actuator. In the air-fuel ratio control in the engine control system, the output signal from the oxygen sensor mounted on the exhaust pipe is used to control the amount of fuel to the engine by the closed loop control mode and thereby to provide a proper air-fuel ratio. In other words, in the conventional engine control system, a three-way catalyst is used to purify the exhaust gas, and the air-fuel ratio of a fuel mixture for purifying at the highest efficiency is controlled to become a stoichiometric air-fuel ratio. The operation of engine at the stoichiometric air-fuel ratio will result in a poor fuel consumption rate and hence uneconomical operation.
Thus, to cope with the recent exhaust gas regulation and improve the rate of fuel consumption of an engine, the air-fuel ratio is made to be lean in accordance with the driving condition of the engine, for example, upon deceleration as is well known.
In this case, the air-fuel ratio is corrected to increase by a predetermined rate relative to a certain fixed air-fuel ratio, or the stoichiometric air-fuel ratio. However, since the characteristics of the fuel system are changed for each engine and undergo secular variation, the stoichiometric air-fuel ratio can not be always obtained and the corected air fuel ratio is not always proper from the standpoint of the fuel consumption rate and exhaust gas purification.